Protection of traffic in optical networks can generally be classified under two types. The first type of protection, being a concept of line protection, is a so-called Optical Multiplex Section (OMS) protection used preferably in multi-channel multi-section ring networks. The ring network is formed by at least two concentric optical fiber rings capable of carrying data traffic in two opposite directions (clockwise and counterclockwise) between network elements NE in the ring. The two concentric fiber rings usually serve as a main route and a protection route for all optical channels of the network, although sometimes the main and the protection routes are arranged within the same fiber. The OMS protection ensures that in case of a fault of the traffic via a multiplex section on the main route (say, in a section between two specific network elements, that may include multiple optical channels), the protection route can be used to redirect the required data traffic from the main route, and thus to bring the traffic to its destination network element using the opposite direction. In this case, the redirected traffic might pass a longer distance than it would pass via the main route. The OMS protection is usually utilized in SDH and SONET optical networks, where it is respectively named MSSPRING (Multiplex Section Shared Protection Ring) and BLSR (Bi-directional Line Shared Protection).
The second known type of traffic protection in optical networks is a so-called OCH protection (Optical Channel protection). The OCH protection is intended for protecting a specific optical channel and by default can be implemented by providing simultaneous transmission of data traffic of that specific optical channel along both the main route and the protection route. In case of a fault in one of the routes, the other one will safely bring the traffic to its destination point.
Some attempts to combine the OCH and the OMS protection concepts are known in the art.
An article “VDM architectures and economics in Metropolitan areas” to R. H. Cardwell et al in the Optical Networks Magazine July 2000 describes a number of possible architectures combining both the OMS and the OCH protection principles, though it does not describe methods of how they are used together.
ITU-T standard recommendations G.798 (sections 10.4.1, 12.1.1.1) and G.841 (section 8.4) describe a possibility of implementing both of the above-mentioned protection concepts, and define so-called switch initiation criteria. These criteria are necessary when protection means are activated, for selecting a signal to be further used in the ring network. The switch initiation criteria are based on analyzing presence of various alarm indications in the signals to be compared in their binary electrical form. In addition, complex timing issues are considered for adequate selection criteria.
U.S. Pat. No. 6,317,426 describes a hybrid protection circuit for selectively protecting data streams according to one of the above-mentioned methods. Namely, the hybrid protection circuit can be electrically configured to one of various industry-standard protection techniques, including a bi-directional line-switched ring (BLSR) protection, a unidirectional path switched ring (UPSR) protection, and one-plus-one (1+1 or OCH) line protection. It should be emphasized that the solution of the U.S. Pat. No. 6,317,426 allows using different protection schemes separately.
Other methods exist in the prior art, for example a method described in an article “Optical. Network Architectures: Routing and Protection” by Antonio J. Ramos (CPRM-Marconi) http://www.eurescom.de/public-seminars/1998/)ADM/Proceeedings/Paper16.html. The article proposes using, in ring networks, OMS protection with OCH protection, wherein the latter is performed by electrical digital cross-connects. In case of a network fault, the traffic in the OCH level is rerouted electrically via existing connections by SDH equipment.
To the best of the Applicants' knowledge, the relevant prior art does not describe a method by which a hybrid OMS and OCH protection schemes could be combined in a fully optical manner, at any desired extent (for any number of optical channels as needed), without additional optical network components such as optical cross-connects, and without any form of wavelength conversion.